The present invention relates to a method for exciting Lamb waves in a plate, in particular a container wall, and an apparatus for carrying out the method and for receiving the excited Lamb waves.
The use of Lamb waves in level measurement devices is known per se. The Lamb waves caused in a container wall by a Lamb wave exciter In the form of an electroacoustic transducer propagate as a cyclically sinusoidal deformation on both surfaces and can be picked up by a Lamb wave receiver provided at a distance away on the same container wall, and can be converted back into an electrical signal, provided the Lamb waves can propagate without any impediment. If, on the other hand, the propagation of the Lamb waves is attenuated, which is the situation when the container contents reach a specific level in the region of the level measurement device, then the signal is nonexistent or is at least so heavily attenuated that it can be used as an indication of the level reached.
Based on the fact that Lamb waves are composed of shear waves or transverse waves oscillating at right angles to the surface and of compression or longitudinal waves oscillating tangentially, Lamb waves can be initiated in a plate or container wall of certain elasticity by means of ultrasound waves striking this plate or container wall obliquely.
According to one known method, these ultrasound waves which strike the plate or container wall obliquely can be produced with the aid of a prism or a prismatic molding, which is mounted on the plate and on whose incline a piezoelectric element is firmly arranged and is connected via electrodes to an AC voltage source. The Longitudinal wave (ultrasound wave) initiated in the piezoelectric element by an AC voltage pulse is transmitted through the prism to the plate, and strikes this plate a an angle which depends on the included angle 2 of the prism. The proportionality of the ratio of longitudinal waves and transverse waves to the resultant Lamb waves, and thus their amplitudes, is then also dependent on the included angle 2 of the prism. Waves of different orders can be excited depending on the thickness D of the plate or of the container wall and on the excitation frequency xcfx89, of which, however, only the zero order (zeroth mode) waves reach the surface and result in a deformation of the surfaces which propagates cyclically and sinusoidally and may be symmetrical or antisymmetrical with respect to the center plane. Zero order symmetrical Lamb waves are of particular importance for practical use, and these are referred to as the s0 mode (zeroth symmetrical mode). (ULTRASONIC TECHNOLOGY, A Series of Monographs, General Editor Lewis Balamuth, Cavitron Corporation, New York, N.Y., RAYLEIGH AND LAMB WAVES, Physical Theory and Applications, I.A. Viktorov, pages 69, 70 and page 82 paragraph 3).
Special prisms are thus used in order to determine and/or to adjust the phase velocity of the relevant Lamb wave and the wavelength on the surface. Furthermore, in this method for Lamb wave excitation, the size or the dimension of the so-called excitation zone on the boundary surface between the prism and plate or container wall is important, and is governed by the projection of the area covered by the piezoelectric element on the incline of the prism onto the boundary surface between the prism and plate or container wall. The magnitude of the maxima of the excited Lamb waves is directly proportional to this excitation zone (page 84, paragraph 2, pages 87 to 88 loc. cit.).
An apparatus for level monitoring in a container, which makes use of this method, is described in DE 689 03 015 T2.
Another method uses the cyclic structures on the effective surface of a Lamb wave exciter resulting from piston vibration. In this case, compressions are produced at specific points on the plate or on the container wall, and their cyclic separation is governed by the phase velocity of the corresponding Lamb wave.
One problem in the excitation of Lamb waves in a plate or container wall by means of prisms is the impedance matching at the contact surfaces between the prism and the container wall, in which context it must be remembered that the containers whose level is intended to be monitored may be composed of widely differing materials, such as steel, brass, aluminum or else glass etc. When sudden impedance changes occur between the prisms of the Lamb wave exciter or Lamb wave receiver and the container wall, interference effects can occur in the form of parasitic waves or harmonics. The characteristics of the prisms, which are normally manufactured from plastic, preferably a polymer, may vary within wide limits, depending on the density, the concentration of the glass phase, the flow during pressing etc., so that the prisms must be specially matched to each application. Furthermore, certain characteristics of plastic prisms are dependent on temperature to an extent which cannot be ignored. Air holes which occur in the material during production of plastic prisms can undesirably influence the characteristics of the prism. From all this, it follows that production of plastic prisms is highly complex and costly.
One object of the invention is to provide a method for producing Lamb waves in a plate or container wall and an apparatus for carrying out this method, in which method there is no need for a prism and in which the phase velocity of the Lamb waves which are produced and the wavelength can be adjusted and matched easily, for example in an appropriate manner for different plate or container wall materials in which the Lamb waves are intended to propagate.
According to the present invention, this object is achieved in that an IDT transducer (interdigital transducer), which is known per se and comprises a layer composed of piezoelectric material and on one of whose surfaces two electrodes, which engage in one another like fingers, are applied, is coupled acoustically to the plate or container wall by the surface facing away from the electrodes, and a radio-frequency AC voltage is applied to the connections of the electrodes, which AC voltage causes cyclically occurring material compressions and rarefactions, so-called thickness vibration, in the piezoelectric layer between the electrode fingers, in which the speed of propagation or the speed of sound of the longitudinal waves resulting from this in the piezoelectric layer is matched to the phase velocity with which an s0 mode Lamb wave propagates in the material of the plate or of the container wall, in that the distance D between the fingers of the electrodes is selected such that it is equal to half the wavelength xcex/2 of the longitudinal wave to be initiated in the piezoelectric layer.
According to the novel method, no prism is required to excite Lamb waves in a plate or container wall, so that all the disadvantages associated with such prisms are also avoided, such as the material-related temperature dependency or negative influences caused by air holes that occur in the plastic during production of the prisms.
Owing to the relatively good match between the speed of propagation or speed of sound of the longitudinal waves in the piezoelectric layer and the phase velocity of the s0 mode Lamb wave in the plate or container wall, the acoustic coupling between the piezoelectric layer and the plate or container wall produces only an s0 mode Lamb wave in this plate or container wall after activation of the IDT transducer. Parasitic interference waves or harmonics are thus prevented.
The operating frequency of the IDT transducer as a Lamb wave exciter must be matched to the distance D between the electrode fingers and may thus also be varied, that is to say matched to different plate or container wall materials, without any need for changes to be made to the dimensions of the piezoelectric layer itself. Matching by varying the distance between the electrode fingers of an IDT transducer is considerably easier and more economic than by varying the included angle of the prism in the known prismatic Lamb wave exciters. In this case, the measured effectiveness of the IDT transducer as a Lamb wave exciter is comparable to that of the conventional Lamb wave exciters, in which a piezoelectric layer ""s used to excite the Lamb waves on a polymer prism having a specific included angle.
The matching of the speed of propagation or speed of sound of the thickness vibration in the piezoelectric layer and of the longitudinal waves resulting from this to the phase velocity of the s0 mode Lamb wave in the material of the plate or container wall by varying the distance D between the fingers of the electrodes of the IDT transducer can be carried out by FEM simulation (finite element method).
An apparatus for carrying out the invention comprises an IDT transducer (interdigital transducer) which is known per se and for its part comprises a piezoelectric layer on one of whose surfaces two electrodes, which engage in one another like fingers, are applied; the surface of this IDT transducer facing away from the electrodes is acoustically coupled to the plate or the container wall, and the connections of the electrodes are connected to a radio-frequency AC voltage source. The distance between the fingers of the two electrodes is in this case equal to half the wavelength xcex/2 of the thickness vibration or longitudinal wave in the piezoelectric layer caused by the piezoelectric effect when the radio-frequency AC voltage is applied; the speed of propagation or speed of sound of the thickness vibration in the piezoelectric layer, and of the longitudinal waves resulting from this, is thus matched to the phase velocity of the s0 mode Lamb wave in the material of the plate or of the container wall. A physically identical IDT transducer which is likewise acoustically coupled to the container wall and is at a distance from the IDT transducer which acts as the Lamb wave exciter receives the initiated Lamb waves and converts them back into an electrical signal.
According to a first embodiment of the present invention, the two electrodes of the IDT transducer have at least two electrode fingers, which engage in one another like fingers while maintaining the mutual separation D.
According to another embodiment of the present invention, the two electrodes of the IDT transducer have more than two electrode fingers, which engage in one another like fingers while maintaining the mutual separation D.
The piezoelectric layer of the IDT transducer is preferably composed of a piezoelectric ceramic material or of a piezoelectric composite material.
The construction, production and matching of the Lamb wave exciter according to the present invention are considerably simpler and more cost-effective than in the case of the known prismatic Lamb wave exciters.
The Lamb wave exciter according to the invention can be used in conjunction with a Lamb wave receiver constructed in the same way as a sensor for level measurement devices.
The present invention will be described in more detail in the following text with reference to the attached drawings, by way of example: